1. Field of the Invention
The invention relates generally to the field of global positioning system (GPS) receivers. More particularly, the invention relates to a GPS receiver capable of functioning in the presence of interference such as continuous wave (CW) jamming. The invention thus relates to a GPS receiver of the type that can be termed CW jamming robust.
2. Discussion of the Related Art
GPS receivers are most often considered as tools for location or navigation. However, GPS receivers can also be used to provide time. In particular, the GPS constellation is monitored and controlled by the Department of Defense so that a GPS receiver can be used to extract time traceable to the US Naval Observatory (USNO) which in turn tracks UTC (Universal Coordinated Time) that is maintained in Paris as the international time standard. Consequently a GPS receiver can be used to discipline a local oscillator to provide accurate frequency and can be used to provide a time signal that tracks UTC. We refer to time extracted in such a manner as “GPS time.”
Symmetricom Inc. (San Jose, Calif.) provides a series of products based on GPS for providing accurate time and frequency. For example, Symmetricom products are used by telephone companies to provide accurate timing (frequency) signals. Wireless telephone operators that deploy CDMA (code divisional multiple access) cellular telephony are familiar with Symmetricom timing units that are installed in the base station to provide accurate frequency as well as accurate time, embodied, for example, in a signal designated as 1-PPS (for one pulse per second). In practice the interval between pulses can be something other than 1 second, but the pulse position is related in an absolute sense to GPS time. For example, CDMA base stations often require a pulse corresponding to every even second of GPS time.
When a receiver is deployed, there is often little flexibility as to where the antenna may be mounted. In the case of a cellular base station, the GPS antenna is mounted in reasonable proximity to the cellular antenna. Now the signal level received at the antenna from the GPS satellites is extremely small, of the order of −130 dBm (10−10 milliwatt) and thus the GPS receiver is quite susceptible to interfering signals. The nominal bandwidth (one-sided) of the GPS spectrum is 1.1 MHz centered at the RF carrier which is transmitted at the L1 frequency (1.57542 GHz). An interfering signal outside this band can, in principle, be removed by the filters that delineate the GPS band, either at the L1 frequency itself or in combination with filters at a suitable intermediate frequency (IF). Using (analog) filters in this manner to reduce the impact of interfering signals is well known.
There have been some techniques proposed to date that have addressed the problem of interfering signals that lie within the GPS bandwidth. References [6 through 9] address methods proposed for improved performance in the presence of jamming CW interference. The jamming signal could well be inband. Generally speaking, these methods address the mitigation of the impact of the jamming signal on the delay-locked-loop used for code tracking.
One approach is to use an increased number of bits in the ADC (A/D converter) and rely on the correlation process to mitigate the impact of the jammer. This approach is based on the premise that the correlator will distinguish between the Gold code and a CW (tone), which indeed it does, provided that the power level of the jammer is not so large as to send the ADC into a saturation mode. By increasing the dynamic range of the ADC (with the concurrent increase in number of bits) one would expect jamming immunity.
A second approach that has been proposed is to use an antenna array and develop a synthetic antenna pattern that has a null in the direction from which the CW signal is being received. This method is quite complex and expensive.
A third approach is indicated in Ref [7]. The method described therein is one of “cancellation” rather than “removal.” That is, a “replica” (or as close to a replica as is feasible) of the jamming signal is generated and is subtracted from the received signal. This cancellation method is of value when the jamming signal is indeed a very narrow-band CW signal, essentially a pure tone. For wider applicability, multiple CW “replicas” can be generated and subtracted in the event that there are more than one jamming signals. This method has a severe drawback in the sense that if the replica generated is not indeed identical and 180-degrees out-of-phase from the jamming signal, one could actually be adding interference.